David Weber - More Than Honor

Unfortunately for the Legislaturalists, they soon discovered that conquest was not the solution they had hoped. True, they could loot the economies of conquered worlds, but unless they wanted servile insurrection, there was a limit to how badly they could wreck their subject economies. Worse, the military machine required to conquer and then police their new empire cost even more than they had anticipated, particularly as their alarmed and (so far) unconquered neighbors began to arm in reply. Despite all efforts, their budgets remained stubbornly in the deficit column; they simply could not pay for both their military and the support of their subsidized population out of available resources. There was an appearance of prosperity on the home front, but those in informed positions knew that it was only an appearance. In short, the "Republic" had only two options: continue to expand, or collapse.

And so, in 1900 pd, the People's Republic had no choice but to look for fresh fields to conquer . . . and found, directly in its path, between it and the additional worlds it had to have, a small but wealthy star system known as the Star Kingdom of Manticore.